595
FXUS06 KWBC 181957
PMDMRD
Prognostic Discussion for 6 to 10 and 8 to 14 day outlooks
NWS Climate Prediction Center College Park, MD
300 PM EDT Wed September 18 2024

6-10 DAY OUTLOOK FOR SEP 24 - 28 2024

A strong anomalous ridge stretching from northeastern Canada southward to
southwestward into much of the central and eastern Contiguous United States
(CONUS) is expected to remain approximately in place through the 6 to 10 day
period while gradually weakening. The strength of this ridge will have peaked
just prior to this period, but 500-hPa height anomalies are forecast to average
at least +27 dm over southern Baffin Island, and the Canadian operational model
shows a 576 dm closed 500-hPa height circulation on day 6. To the east and
south, an area of near- or below-normal 500-hPa heights is forecast over and
near the southeastern CONUS. Individual members of the ensembles show the
potential for tropical or subtropical cyclone development, but confidence in
the placement and timing of any system that might develop is low, with a broad
array of possibilities shown. Meanwhile, a mid-level trough over east-central
North America should drift eastward. Todays guidance continues the trend of
keeping this feature intact for longer, tracking it into the eastern CONUS by
the end of the period. The ensemble means are in slightly better agreement than
yesterday on the evolution of this weak mid-level trough, and a compromise of
the guidance was preferred. Farther west, the models are in better agreement
than yesterday as well, although substantive differences remain. The GEFS and
European ensemble means both depict a shortwave through moving into western
Canada, then weakening as it rotates northeastward, while the longwave trough
reintensifies near western Alaska. The Canadian ensemble mean is different,
keeping the initial shortwave farther west and deamplifying the trough more
quickly. The majority solution is preferred, pulling an initial shortwave
farther east than anticipated yesterday, then reloading the trough farther
west. The initial shortwave amplifies a mid-level ridge to its southeast over
west-central North America which weakens but remains identifiable through the
forecast period before settling over the Rockies or Plains. Overall, these
changes result in a somewhat less amplified pattern across North America by day
10, but with strong positive height anomalies still centered in northeastern
Canada, a mid-level trough affecting Alaska, flat ridging with above-normal
500-hPa heights in the central and western CONUS, and a very weak mid-level
trough farther east. Near-normal 500-hPa heights are forecast for Hawaii, with
a weak cyclonic circulation situated west of the state that slowly dissipates
through the period.

Despite uncertainties in the evolution of unsettled weather in the Southeast,
the tools derived from the ensemble means are in fair to good agreement, but
with some notable exceptions. In addition to differences in the model
solutions, additional uncertainty comes from inconsistencies between the raw
and bias-corrected temperature and precipitation output on one hand, and the
calibrated and reforecast data on the other. There is general agreement that
above-normal temperatures will prevail across most of the CONUS, with the best
chances for above-normal temperatures across parts of the Great Lakes and
interior Northeast. Significantly enhanced odds for above-normal temperatures
also cover parts of the interior West, consistent with the forecast for a
stronger and longer-lasting mid-level ridge than anticipated in yesterdays
forecast. Meanwhile, near-normal temperatures are favored in parts of the
Eastern Seaboard, which is consistent with the presence of surface high
pressure over southeastern Canada or the Northeast bringing in mild, maritime
air on persistent easterly winds. The reforecasts are warmer here than the raw
and bias-corrected tools, but the cooler solutions are more consistent with the
mid-level pattern and the forecast location of surface high pressure. In
addition, clouds and precipitation in and near the southeastern CONUS have the
potential to keep temperatures down. There is also considerable uncertainty
across Alaska, where raw and bias-corrected tools show colder than normal
conditions covering most or all of the state while the reforecast and
calibrated outputs lean toward warmer than normal weather in southeastern
Alaska and the eastern Mainland. With good agreement between the GEFS and
European ensembles, there is increased confidence in a scenario which brings
one storm system across southeastern Alaska early in the period, followed by
another system approaching from the west, spun up by the reloading mid-level
trough. This would favor a colder solution in central and western Alaska, with
variable temperatures averaging near normal farther east. Meanwhile, warmer
than normal weather is favored over the northwestern half of Hawaii, consistent
with the consolidation forecast. The European ensemble mean shows cooler
weather across the islands, but this is not consistent with the expected
mid-level pattern.

In the southeastern CONUS, there is a lot of uncertainty regarding the
formation and evolution of surface low pressure systems, including the
possibility for tropical or subtropical cyclone development. However, most
potential scenarios would result in heavy rain affecting at least part of the
Southeast. Chances for abnormally wet weather exceed 50 percent in the South
Atlantic region, where guidance is in good agreement showing unusually heavy
rain; however, it should be emphasized that there is a large degree of
uncertainty regarding the development and track of any tropical or subtropical
cyclones that form. The likelihood of unusually heavy precipitation also
exceeds 60 percent in southeastern Alaska, where two impactful storm systems
are expected during the period. Elsewhere, enhanced chances for surplus
precipitation extend northward up the Atlantic Seaboard into lower New England,
across the Ohio, Tennessee, and Middle and Lower MIssissippi Valleys, and the
south-central Plains, consistent with most of the derived precipitation tools.
Farther west, the mid-level ridge pushing into the northwestern and eventually
the north-central CONUS should inhibit precipitation development, resulting in
a large area with increased chances for subnormal precipitation extending as
far south as the southern Rockies. The western half of the Gulf Coast is an
area of uncertainty. Raw ensemble mean outputs from the GEFS and European
models favor lighter than normal rainfall, particularly over Texas, while
reforecast tools call for wetter conditions. Since this area could be affected
with a more westward track of potential tropical systems, the forecast splits
the difference, favoring above-normal precipitation in the Lower Mississippi
Valley, and neither precipitation extreme farther west. Meanwhile, weak
cyclonic mid-level flow increases the odds for surplus precipitation over
western Hawaii while the consolidated forecast along and many deterministic
models lean toward drier than normal conditions in the Big Island.

The official 6-10 day 500-hPa height blend consists of 15% of Today`s 6z GFS
Ensemble Mean centered on Day 8, 25% of Today`s GFS Superensemble Mean centered
on Day 8, 10% of Today`s operational 6z GFS centered on Day 8, 25% of Today`s
0z European Ensemble Mean centered on Day 8, 10% of Today`s operational 0z
ECMWF centered on Day 8, and 15% of Today`s 0z Canadian Ensemble Mean centered
on Day 8


FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD: Average, 3 out of 5, with fair
agreement on the evolution of the mid-level pattern and areas of where raw and
derived temperature and precipitation tools are in poor agreement.

8-14 DAY OUTLOOK FOR SEP 26 - OCT 02, 2024

Deamplification of the mid-level pattern is expected during week-2 is expected,
with mostly a zonal pattern covering the North Pacific Ocean and North America
by the end of the period. The moderately amplified pattern in place at the
start of week-2 should evolve into broadly near- or above-normal 500-hPa
heights, with weak mid-level ridging over northern and western North America
while a very weak mid-level trough settles into the eastern CONUS. But these
features will be subtle, with 500-hPa heights much closer to normal than at the
start of the 6- to 10-day period. Still, the favorable set-up for storm
development will continue at least through the beginning of week-2, including
possible tropical or subtropical cyclone development.

This pattern should keep above-normal temperatures over most of the CONUS
outside the South Atlantic region and adjacent areas, where surface high
pressure to the north and unsettled weather are expected to keep daytime
temperatures closer to normal. Meanwhile, slightly enhanced odds for warmer
than normal conditions continue over westernmost Hawaii, and the dissipating
mid-level trough also increases the odds for warmer than normal weather in
northeastern Alaska. The only areas with increased chances for subnormal
temperatures are western and south-central Alaska, where subnormal 500-hPa
heights will persist the longest.

Enhanced chances for wet weather continue in southeastern Alaska initially
downstream from a mid-level trough, but the relaxing 500-hPa flow will lower
expected precipitation totals later in the period, so the odds of above-normal
precipitation for week-2 as a whole are lower than in recent forecasts.
However, tools continue to depict unsettled weather, including the potential
development of a tropical or subtropical cyclone, over the southeastern CONUS.
Therefore, enhanced chances for above-normal rainfall still exceed 50 percent
in the South Atlantic region from the central Carolinas southward.
Precipitation guidance derived from the deterministic models bring increased
chances for surplus precipitation farther north today, so the area where wetter
than normal weather is favored extends northward into the Northeast and lower
New England. Guidance remains inconsistent in the central Gulf Coast region,
but odds lean toward near- or above-normal precipitation in the official
forecast because there is a nontrivial chance that a more westward track of
storms forecast across the Southeast, may influence conditions over the central
and western Gulf of Mexico as well. Meanwhile, drier than normal weather is
favored across a large area covering most of the central and western CONUS
under weakening mid-level ridging and nondescript 500-hPa flow. Slightly
increased odds for near normal rainfall remain in place over westernmost
Hawaii, but deterministic model output at the consolidation forecast lean
toward drier than normal weather in the eastern islands, especially on the
southeastern tier of the Big Island.

The official 8-14 day 500-hPa height blend consists of: 40% of Today`s 6z GFS
Ensemble Mean centered on Day 11, 40% of Today`s 0z European Ensemble Mean
centered on Day 11, and 20% of Today`s 0z Canadian Ensemble Mean centered on
Day 11


FORECAST CONFIDENCE FOR THE 8-14 DAY PERIOD: Below average, 2 out of 5, due the
inherent uncertainties involved in forecasting a mid-level pattern change, some
areas of disagreement among the temperature and precipitation tools, and
significant uncertainty on the evolution of the wet pattern in the Southeast
and perhaps farther west along the Gulf Coast.

FORECASTER: Rich Tinker

Notes:


Automated forecasts are issued on Saturday and Sunday. Occasionally manual
intervention is necessary to address quality control and consistency issues. In
these cases, forecasts are manually drawn but a full discussion is not issued.


The notation for the categorical forecast indicated on the maps is the same as
that in the tables: A-above   N-near normal   B-below


The temperature map shows regions with > 33% chance of being warmer (orange,
"A"), colder (blue, "B"), or close to (unshaded, "N"). Historical average
values for the calendar period of the forecast (dashes, "f").  Labels on the
shaded lines give the probability (> 33%) of the more likely category (B or A).
 Probability of N is always < 40%.


The precipitation map shows regions with > 33% chance of being wetter (green,
"A"), drier (tan, "B"), or close to (unshaded, "n"). Historical median values
for the calendar period of the forecast (dashes, "inches").  Labels on the
shaded lines give the probability (> 33%) of the more likely category (B or A).
 Probability of N is always < 40%.


In the southwest and other climatologically dry regions - there will be a
greater than 33.3% chance of no precipitation and occasionally even a normal
(i.e. Median) value of zero - especially during the dry seasons.  In such cases
a forecast of near normal is effectively a forecast of little or no
precipitation.


The climate prediction center uses 1991-2020 base period means as reference in
the climate outlooks.

The next set of long-lead monthly and seasonal outlooks will be released on
September 19.


Analogs to the 5 day mean observed pattern centered 3 days ago (D-3)
for the region from 20N to 70N latitude and 175E to 60W longitude
include the 5 day periods centered on the following dates:
20050905 - 20040920 - 20070923 - 20070929 - 19880919


Analogs to the 7 day mean observed pattern centered 4 days ago (D-4)
for the region from 20N to 70N latitude and 175E to 60W longitude
include the 7 day periods centered on the following dates:
20070922 - 20070927 - 20040919 - 19830930 - 20050905


6-10 DAY OUTLOOK TABLE
Outlook for Sep 24 - 28 2024

STATE      TEMP PCPN   STATE      TEMP PCPN   STATE      TEMP PCPN
WASHINGTON  A    B     OREGON      A    B     NRN CALIF   A    B
SRN CALIF   A    N     IDAHO       A    B     NEVADA      A    B
W MONTANA   A    B     E MONTANA   A    B     WYOMING     A    B
UTAH        A    B     ARIZONA     A    B     COLORADO    A    N
NEW MEXICO  A    N     N DAKOTA    A    B     S DAKOTA    A    B
NEBRASKA    A    N     KANSAS      A    A     OKLAHOMA    A    A
N TEXAS     A    N     S TEXAS     A    N     W TEXAS     A    N
MINNESOTA   A    B     IOWA        A    N     MISSOURI    A    A
ARKANSAS    A    A     LOUISIANA   A    A     WISCONSIN   A    N
ILLINOIS    A    A     MISSISSIPPI A    A     MICHIGAN    A    N
INDIANA     A    A     OHIO        A    A     KENTUCKY    A    A
TENNESSEE   A    A     ALABAMA     A    A     NEW YORK    A    A
VERMONT     A    N     NEW HAMP    A    N     MAINE       A    N
MASS        A    A     CONN        A    A     RHODE IS    N    A
PENN        A    A     NEW JERSEY  N    A     W VIRGINIA  A    A
MARYLAND    N    A     DELAWARE    N    A     VIRGINIA    N    A
N CAROLINA  N    A     S CAROLINA  N    A     GEORGIA     N    A
FL PNHDL    N    A     FL PENIN    A    A     AK N SLOPE  B    A
AK ALEUTIAN B    A     AK WESTERN  B    N     AK INT BSN  B    A
AK S INT    B    A     AK SO COAST B    A     AK PNHDL    N    A



8-14 DAY OUTLOOK TABLE
Outlook for Sep 26 - Oct 02, 2024

STATE      TEMP PCPN   STATE      TEMP PCPN   STATE      TEMP PCPN
WASHINGTON  A    B     OREGON      A    B     NRN CALIF   A    B
SRN CALIF   A    N     IDAHO       A    B     NEVADA      A    B
W MONTANA   A    B     E MONTANA   A    B     WYOMING     A    B
UTAH        A    B     ARIZONA     A    B     COLORADO    A    B
NEW MEXICO  A    B     N DAKOTA    A    B     S DAKOTA    A    B
NEBRASKA    A    B     KANSAS      A    B     OKLAHOMA    A    B
N TEXAS     A    N     S TEXAS     A    N     W TEXAS     A    B
MINNESOTA   A    B     IOWA        A    B     MISSOURI    A    B
ARKANSAS    A    N     LOUISIANA   A    A     WISCONSIN   A    B
ILLINOIS    A    B     MISSISSIPPI A    A     MICHIGAN    A    B
INDIANA     A    N     OHIO        A    N     KENTUCKY    A    A
TENNESSEE   A    A     ALABAMA     N    A     NEW YORK    A    N
VERMONT     A    N     NEW HAMP    A    N     MAINE       A    N
MASS        A    A     CONN        A    A     RHODE IS    A    A
PENN        A    A     NEW JERSEY  A    A     W VIRGINIA  A    A
MARYLAND    N    A     DELAWARE    N    A     VIRGINIA    N    A
N CAROLINA  N    A     S CAROLINA  N    A     GEORGIA     N    A
FL PNHDL    N    A     FL PENIN    A    A     AK N SLOPE  N    A
AK ALEUTIAN B    N     AK WESTERN  B    N     AK INT BSN  N    A
AK S INT    N    A     AK SO COAST B    A     AK PNHDL    B    A

                           LEGEND
TEMPS WITH RESPECT TO NORMAL     PCPN WITH RESPECT TO MEDIAN
A - ABOVE   N  - NEAR NORMAL     A - ABOVE   N - NEAR MEDIAN
B - BELOW                        B - BELOW

THE FORECAST CLASSES REPRESENT AVERAGES FOR EACH STATE. NORMAL
VALUES - WHICH MAY VARY WIDELY ACROSS SOME STATES - ARE
AVAILABLE FROM YOUR LOCAL WEATHER SERVICE FORECAST OFFICE.

FOR ADDITIONAL INFORMATION SEE MESSAGE FXUS06 KWBC - ON AWIPS AS
PMDMRD.

$$